1. Field of the Invention
The present invention relates to a display device substrate and, more particularly, to a display device substrate effectively used in a liquid crystal display device or the like having a large display screen and a method of manufacturing the display device substrate.
2. Description of Related Art
In recent years, display devices represented by a liquid crystal display device and an EL (electroluminescence) display device have been used in various fields as display devices for personal computers or word processors, TV display devices, and projection type display devices because of the characteristics such as small thickness, light weight, and low power consumption of the display devices.
Of these display devices, an active-matrix display device in which switching elements are electrically connected to pixel electrodes provides a preferred display image free from crosstalk between adjacent pixels. For this reason, the active-matrix display device is extensively researched and developed.
The transmissive type active-matrix liquid crystal display device will be briefly described below with reference to its structure. This active-matrix liquid crystal display device is constructed in the following manner. Alignment films are formed on an array substrate and a counter substrate. Both the substrates are arranged to cause the alignment films to oppose each other, and a liquid crystal composition is held between the substrates. The array substrate is constructed as follows: a plurality of signal lines and a plurality of scanning lines are arranged on a glass substrate in the form of a matrix, thin-film transistors (referred to as TFTs hereinafter) serving as switching elements are arranged near cross points between the signal lines and the scanning lines, respectively, and pixel electrodes made of ITO (Indium Tin Oxide) are provided through the TFTs. In addition, storage capacitor lines are arranged on the glass substrate to be almost parallel to the scanning lines, and an insulating film is interposed between each storage capacitor line and each pixel electrode to form a storage capacitor (Cs) between the corresponding storage capacitor line and pixel electrode.
The counter substrate is arranged as follows: a matrix-like light-shielding film for shielding the TFTs and peripheral portions of pixel electrodes from light is arranged on the glass substrate, and a counter substrate made of ITO is arranged on the light-shielding film through an insulating film.
For example, the signal lines and scanning lines of the array substrate are electrically connected to a drive circuit board through an FPC (Flexible Printed Circuitboard) obtained by forming a metal wiring layer on a polyimide film, TAB (Tape Automated Bonding) obtained by arranging a drive element on a flexible wiring substrate, or the like. The counter electrode of the counter substrate is electrically connected to the array substrate through a transfer obtained by dispersing conductive particles, such as silver particles in a resin, and electrically connected to the drive circuit board through the FPC, TAB, or the like.
A liquid crystal display device or the like having the above arrangement strongly demands high definition and a large display screen. When the device is increased in size to realize a large display screen, the distortion of the waveform of a scanning pulse applied to a scanning line or a video signal voltage applied to a signal line increases as the distance between the corresponding scanning or signal line and a power supply increases. For this reason, display nonuniformity occurs in the display screen. When a drive element is directly mounted on the array substrate, display nonuniformity occurs in the display screen due to the wiring resistance of an electrode wiring layer, arranged on the array substrate, for supplying a drive signal to the drive element. In addition, when a drive circuit portion is integrally formed on the array substrate, due to the wiring resistance of an electrode wiring layer extended on the drive circuit portion, the operation frequency of the drive circuit is limited, or display nonuniformity occurs in the display screen.
In order to solve such technical problems, a structure in which scanning lines are made of a low-resistance material, such as aluminum, has been proposed. However, aluminum easily corrodes, and an uneven surface is easily formed during manufacture of the aluminum scanning lines. For this reason, an aluminum single-layer film cannot be used as the scanning lines. Therefore, Jpn. Pat. Appln. KOKAI Publication No. 6-120503 discloses that a multilayer film obtained by forming a metal film, such as a chromium film, a tantalum film, or a titanium film on an aluminum film is used as the scanning lines.
However, in this structure, after the aluminum is formed and patterned, forming and patterning of the other metal film is required. For this reason, in order to cover the aluminum film with the other metal film, the wiring width of the other metal film must be larger than that of the aluminum film due to patterning accuracy, and the entire wiring width of the other metal film is larger than that of the aluminum film by 6 to 7 microns.
In this manner, although aluminum is used as a wiring material, it is necessary that the wiring width be large in order to achieve a low resistance. In addition, a step difference portion along an aluminum wiring layer is formed on the surface of the wiring layer, and an insulating failure may occur at the step portion in a gate insulating film or an interlevel insulator, thereby degrading reliability of the wiring layer.